Near-infrared to visible and ultraviolet upconversion in TiO2 thin films modified with Er and Yb

Upconversion as a modification strategy to enhance the utilization of sunlight in titanium dioxide photoanodes with an internal upconverter was investigated. TiO2 thin films containing an Er activator and Yb sensitizer were deposited in the magnetron sputtering process on conducting glass, amorphous silica, and silicon. Scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, and X-ray absorption spectroscopy allowed assessment of the thin film composition, structure, and microstructure. Optical and photoluminescence properties were measured by means of spectrophotometry and spectrofluorometry. Changing the content of Er3+ (1, 2, 10 at%) and Yb3+ (1, 10 at%) ions allowed us to achieve thin film upconverters with a crystallized and amorphous host. Upon 980 nm laser excitation Er3+ exhibits upconversion with the main emission in green (2H11/2 → 4I15/2, λem ≈ 525 nm) and weak emission in red (4F9/2 → 4I15/2, λem ≈ 660 nm). For a thin film with a higher ytterbium content (10 at%) a significant increase in red emission and upconversion from NIR to UV was observed. The average decay times of green emission for TiO2:Er and TiO2:Er,Yb thin films were calculated based on time-resolved emission measurements.


Introduction
Lanthanide ions (Ln 3+ ), along with transition metal ions and organic molecules, are well known to be luminescent centers in materials. 1,2 The ladder-like structure of Ln 3+ with abundant 4f energy levels enables multiple intracongurational transitions and allows not only light emission according to Stokes law but also the upconversion (UC) of two or more low-energy photons into a photon of a higher energy in the anti-Stokes upconversion process. 3 Upconversion nanomaterials nd applications in photonics, biomedicine, sensors, photocatalysis, photovoltaics, and photoelectrochemical cells. [4][5][6][7][8][9][10] A typical upconversion material consists of emitting ions (activator), and optionally enhancing ions (sensitizer), embedded in a matrix (host). 8,11,12 Er 3+ ions are one of the extensively studied activator ions for upconversion from nearinfrared (NIR) to visible (VIS), red and green, and ultraviolet (UV) regions (390-410 nm), which is due to an appropriate energy level structure. 11 To enhance the absorption of infrared photons an Yb 3+ sensitizer is added, due to its large absorption cross-section in NIR compared to other Ln 3+ ions (9.11 × 10 −21 cm −2 at 980 nm excitation). 13 When ytterbium ions absorb infrared photons, electrons are excited from the 2 F 7/2 ground state to the higher energetic state. The excited state 2 F 5/2 of Yb 3+ matches the f-f transitions in Ln 3+ activators, for example it overlaps the 4 I 11/2 state of Er 3+ , thus excessive energy can be transferred to activator ions by the energy transfer mechanism. 14 Choice of the host material is crucial due to its impact on the upconversion ability, emission intensity, and wavelength of emitted light. 5,11,12,15 To achieve efficient luminescence, the host should be a low phonon material, which decreases the rate of non-radiative relaxation, with a good distribution of a high amount of activator ions to avoid cross-relaxation mechanisms. Usually, for inorganic upconversion materials uorides (NaYF 4 ), oxides (ZrO 2 , Y 2 O 3 , SiO 2 , TiO 2 ), oxyhalides (GdOCl), oxysuldes (La 2 O 2 S), and phosphates (LuPO 4 , YPO 4 ) are used as hosts. 5 TiO 2 , in polymorphic forms of anatase, rutile and brookite, is an important semiconductor for photocatalysis and photoelectrochemical cells (PEC). [16][17][18] In particular, its ability to utilize solar energy to split water into hydrogen and oxygen is applicable for green solar energy. However, the wide bandgap of TiO 2 (E g, rutile = 3.0 eV, E g, anatase = 3.2 eV) limits light harvesting to UV, while VIS and NIR light are underutilized (sub-bandgap energy photons) and thus modication strategies such as microstructure modication, doping, formation of heterostructures, bandgap engineering, and enhancement by the upconversion process are investigated. 16,17 The last mentioned strategy does not rely on the modication of TiO 2 properties to better t the solar spectrum, but rather on the change of the light distribution. Combining TiO 2 with UC materials allows to absorb additional photons from the NIR range, then upconvert them to visible and UV that can excite TiO 2 .
To the best of our knowledge, the application of upconverters in photoelectrochemical cells for water splitting, contrary to well-developed upconverters for photovoltaic cells or photocatalysts, is still an unexploited research area. 17,19,20 Moreover, most of the investigated UC materials are in the form of powders 21 or bulk materials, 22 like optical bers, while upconversion thin lm materials remain a challenge. To achieve upconversion and maintain good photoelectrochemical performance in case of a few hundred nanometers thick TiO 2 lm photoanode, it is important to nd a balance between introducing a high amount of activator and sensitizer ions, their distribution in the host, and obtaining proper structure and microstructure of TiO 2 . In general, a higher activator concentration increases the possibility of upconversion, but aer reaching a certain level, the activator ions are too close to each other and the UC efficiency is lowered due to the crossrelaxation mechanism. Meanwhile, the sensitizer ions need short distances between the ions, which allows for non-radiative energy transfer. Thus, another strategy is to keep the activator ion concentration at a low level, but with the addition of a sensitizer. In that way, the amount of activator ion can be reduced to a few mol%, while for the sensitizer it can reach a high concentration (z20 mol%). 17 Salhi and Deschanvres 23 prepared crystallized TiO 2 powders with 5 mol% Er and 10 mol% Yb via hydrothermal method. However, a high concentration of Ln 3+ ions, due to signicantly larger ionic radii than Ti 4+ , leads to the amorphization of TiO 2 . Pérez et al. 24 investigated TiO 2 lms with Er 3+ ions doping concentration up to 10 at%, for which diffraction peak of (1 0 1) anatase plane was still observable, but the amorphous TiO 2 phase was dominant. Johannsen et al. 25 showed that changes in host crystallinity in Er 3+ doped TiO 2 lms deposited on Si (100) substrates signicantly modied its luminescence properties. Moreover, red-togreen emission ratio relate to Er 3+ /Yb 3+ concentration ratio as presented by Jung, 26 while blue or UV light emission is observed only for UC materials with high efficiency due to low probability of the three or four photon UC processes. 9 In our previous works, we investigated both the concepts of external 27 and internal upconverter, 28,29 and we demonstrated the possibility to use TiO 2 thin lm photoanodes modied with rare earth ions (Nd 3+ , Er 3+ , Yb 3+ ) in photoelectrochemical cells. 27,29 Thin lm photoanodes deposited at 20% O 2 /(O 2 + Ar) ow rate ratio exhibit the best photoelectrochemical properties (high value of photocurrent and rectangular shape of current density versus voltage curves). 27 In our research on photoanodes with internal upconverter, we examined two strategies: introduction of high activator content (TiO 2 :Er (10 at%)) or incorporation of the low activator content with the addition of sensitizer (TiO 2 :Er,Yb (1 at%, 1 at%) and TiO 2 :Er,Yb (2 at%, 10 at%)). Recently, we described the photoelectrochemical properties of such photoanode materials. 29 Here, we would like to focus on the investigation of TiO 2 modied with Er and Yb thin lms luminescence properties and the inuence of the surrounding host on the lanthanide ions upconversion ability. We compare thin lm upconverters with crystallized and amorphous host, which was achieved by changing the content of Er 3+ and Yb 3+ ions.

Experimental
TiO 2 , TiO 2 :Er and TiO 2 :Er,Yb thin lms were deposited in reactive radio frequency magnetron sputtering in an ultra-high vacuum system (PREVAC). The base vacuum in the process chamber, aer heating up the substrates to 350°C, was at the level of 10 −7 -10 −8 mbar. During deposition, the argon and oxygen ow rates were set at 32 sccm and 8 sccm, respectively to keep 20% O 2 /(O 2 + Ar) ow rate ratio, and the working pressure at the level of 6-7 × 10 −3 mbar. Sputtering of 2 ′′ targets (Kurt Lesker Ltd) with diverse compositions, Ti, Ti/Er (90/10 at%), Ti/ Er/Yb (98/1/1 at%), and Ti/Er/Yb (88/2/10 at%), was performed with a power density of 9-10 W cm −2 . More details of process steps and parameters are described in our previous work. 28 Thin lms were deposited on various substrates: amorphous silica (Q) for investigation of the thin lm structure, and to assess the impact of rare earth ions on the optical bandgap of the TiO 2 host, silicon (Si) for observation of the thin lm microstructure and further investigation of its structural properties, conducting indium tin oxide glass (ITO) to assess luminescence properties of upconversion photoanodes.
The average thickness of the deposited lms was measured with a Rank Taylor Hobson Talystep stylus surface prolometer. The structure of the deposited thin lms was determined by means of grazing incidence X-ray diffractometry (GIXRD) with a Philips X'Pert MPD diffractometer (l Cu = 0.154 nm). The diffractograms, with a subtracted background that originates from the silica substrate, were analysed according to data from the International Centre for Diffraction Data (ICDD) database (anatase 01-078-2486 and rutile 01-086-0147 cards). Scanning electron microscopy (SEM) imaging with energy dispersive spectroscopy (EDS) analysis was performed using Thermo Fischer Scientic Helios G4-PFIB-CXe microscope. For further structural investigation, X-ray absorption spectroscopy (XAS) measurements were conducted at SOLARIS National Synchrotron Radiation Centre in Kraków, Poland. XAS spectra were collected with synchrotron so X-ray radiation at the PIRX beamline (Premiere InstRument for XAS, former PEEM/XAS). A more detailed description of the experimental procedure at the PIRX beamline is described in ref. 30. Measurements were carried out with 1 s per point acquisition time and energy steps of 0.05 eV for the pre-edge and white line range, and 0.1 eV over the range free from any sharp absorption peaks. Transmittance (T) and reectance (R) spectra were measured with the UV-VIS-NIR Perkin Elmer double beam Lambda 19 spectrophotometer. Triax 180 Jobin Yvon grating monochromator with 450 W xenon lamp equipped with optical ber and lens was used as a light source for photoelectrochemical experiments. Current versus time characteristics (I-t) were recorded under monochromatic illumination, changing the wavelength from 300 to 420 nm with a 10 nm step. 0.8 M Na 2 SO 4 solution and a three electrode setup (working electrodesample, auxiliary electrodeplatinum foil covered with platinum black, reference electrode -Ag/AgCl in 3 M KCl) was used. The MTM Anko M161E electrochemical analyser served as a potentiostat for I-t measurements. Raman and photoluminescence spectra were collected with the WITec Alpha 300M+ spectrometer equipped with a 488 nm laser, the Zeiss LD EC 50× objective, and a CCD UV-NIR detector. Measurements in the 0-15 000 cm −1 wavenumber range were conducted in multiple scans, while each scan consisted of 2 seconds acquisition and 2 accumulations. Upconversion steady-state luminescence spectra and timeresolved emission measurements were performed in Horiba Fluorolog-QM-75-12C spectrouorometer equipped with 700 mm double monochromator and photon counting PMT 920 IS detector with thermoelectric cooling, upon 980 nm semiconductor laser diode (LD) excitation.

Results and discussion
Thin lm samples are named according to the composition of the sputtering targets. The list of samples with target composition, temperature of substrate during deposition, deposition rate, and thickness measured with the prolometer can be found in Table 1.  To investigate the structure, microstructure and determine the elemental composition of thin lms, GIXRD, SEM and EDS measurements were conducted (Fig. 1). Analysis of top-view SEM images and diffractograms showed that a high addition of Er and Yb ions caused amorphization of thin lms, a decrease in average grain size, and a smooth surface compared to crystallized pure TiO 2 or thin lm with a low addition of Er and Yb. Crystallized TiO 2 and TiO 2 :Er,Yb (1 at%, 1 at%) lms have grains with pyramid-like tops with average grain size 57 and 52 nm, respectively. Anatase is a dominant phase, whereas rutile and a trace of brookite phase are also observed. For amorphous TiO 2 :Er (10 at%) the average grain size was 26 nm and for TiO 2 :Er,Yb (2 at%, 10 at%) 18 nm. It was reported that a high concentration of Er 3+ seems to stabilize the TiO 2 amorphous phase or hinder the crystallization. 31 The ionic radii of Er 3+ and Yb 3+ , 89 pm and 86 pm respectively, are signicantly larger than Ti 4+ ionic radius (68 pm). This mismatch has an impact on lattice distortion of TiO 2 . 20,32,33 Therefore, substitution of Ti 4+ in its lattice site is less probable than interstitial incorporation or segregation of Ln 3+ ions at the grain boundaries. 20 The EDS elemental analysis results (Fig. 1), considering the semi-quantitative character of this technique, correlate well with the composition of the targets used for the sputtering. In all EDS spectra, due to the beam penetration depth larger than lm thickness, besides peaks from elements in the thin lm (Ti, O, Er, and Yb), a high-intensity peak originating from the substrate (Si) is observed.
The presence of Er and Yb ions in lms was also conrmed in the XAS measurements (Fig. 2). The absorption of synchrotron X-ray radiation in total electron yield (TEY) mode, as a very sensitive method, allowed to easily detect even very low addition of Er and Yb. The M 4,5 Er and M 5 Yb lines of thin lms and reference oxide powders (Er 2 O 3 , Yb 2 O 3 ) are presented in Fig. 2c and d. The area under the peak line in the XAS spectra collected for the same measurement sensitivity is dependent on the detected ions concentration. To compare the shape of the spectrum curves, it is necessary to perform a normalization procedure. The normalization procedure for the Ti L 2,3 spectra was described in detail in our recent work, 30 and here an analogous approach was applied for Ti L 2,3 , O K, Er M 4,5 and Yb M 5 spectra. The normalized Er M 4,5 XAS spectra (Fig. 2c) have a similar shape, not affected by the phase composition of the samples. The same can be noticed for the normalized Yb M 5 spectra (Fig. 2d). The Er M 5 and M 4 lines are related to 3d 5/2 / 4f and 3d 3/2 / 4f transitions with major absorption energies at 1404.9 and 1446.3 eV, respectively as reported in ref. 34. The collected spectra showed peak splitting of the Er M 5 line into four maxima (1404, 1406, 1409, and 1411 eV), while for Er M 4 one maximum at 1447 was observed. For Yb M 5 only a single line was observed at 1521 eV, which can be attributed to 3d 10 4f 7/2 13 / 3d 5/2 9 4f 14 transition. 34 Spectra in Fig. 2c and d insets are presented without normalization but for the same measurement sensitivity. Here, differences in the intensity of the signal are related to the amount of ions in the samples. The Ti L 2,3 and O K lines are strongly affected by changes in phase composition ( Fig. 2a and b). The Ti L 2,3 spectrum with sharp peaks at the pre-edge, and the splitting of the L 3 -e g peak at 460 eV indicated a well crystalized anatase and rutile phase, while for amorphous samples the L 3 -e g peak splitting did not occur (inset in Fig. 2 a). For detailed analysis of Ti L 2,3 lines, see ref. 30. The maxima of the O K lines at 531, 533, 539 and 544 eV can be assigned to 1s / t 2g , 1s / e g , and 1s / a 1g ,t 1u transitions (see also ref. 35). The O K line for the crystallized sample ( Fig. 2b) had better split t 2g , e g , a 1g , and t 1u peaks than the O K spectra of the amorphous lms presented in the inset. The spectral dependence of transmittance coefficient (T) (Fig. 3a) showed oscillations of transmittance in the weak absorption region, which are due to the interference in thin lms. The thin lm thickness ranges from 220 to 340 nm as presented in Table 1 and Fig. 3b-f. As a reference, an additional TiO 2 lm was usedamorphous TiO 2 lm deposited at room temperature, which preparation and characterization were described in detail in our recent work. 36 For both TiO 2 and rare earth modied TiO 2 amorphous thin lms, a shi of the fundamental absorption edge to higher energies in comparison to crystallized lms can be observed. The absorption coefficient (a) in the strong and weak absorption region was calculated using the "envelope method" from the transmittance and reectance spectra taking into account the values of the thin lm thickness measured with the prolometer. The optical bandgap E g can be determined from the Tauc method based on eqn (1): where a is the absorption coefficient, ħ is the Planck constant, n is the photon's frequency, factor g is equal 2 for the indirect fundamental optical transition, A is a constant, E g is the bandgap energy. 37 The optical bandgap E g values determined from the cut-off of the (a$ħn) 1/2 versus ħn dependence are presented in Fig. 3b-f. More details about this procedure can be found in our work on TiO 2 :Er thin lms. 28 Here, as references, crystallized (Fig. 3b) and amorphous (Fig. 3c) TiO 2 thin lms, with bandgap of 3.27 eV for nanosized anatase and 3.35 eV for amorphous TiO 2 , are shown. Rare earth modied lms (Fig. 3df) have E g = 3.33, 3.46, and 3.63 eV for TiO 2 :Er,Yb (1 at%, 1 at%), TiO 2 :Er (10 at%), and TiO 2 :Er,Yb (2 at%, 10 at%), respectively. The blue shi of TiO 2 :Er,Yb and TiO 2 :Er bandgap values could be associated with thin lm amorphization or increased amount of Ln 3+ ions. Determination of the bandgap energy from the Tauc method allows to factor in character of the material in the form of a thin lm (dominant role of the thickness, and oscillations in transmittance and reectance spectra) independently of the material functional properties. The bandgap energy for photoanode materials can be also evaluated from photoelectrochemical methods. 38 For a n-type lm, if lm thickness L f is very small, the photocurrent I ph and the absorption coefficient a relationship could be simpli-ed to eqn (2): 39 where e is the electronic charge and J 0 is incident light ux.
Combining eqn (1) and (2), we achieve eqn (3): For TiO 2 and TiO 2 :Er,Yb (1 at%, 1 at%) thin lm photoanodes, which are samples with the best photoresponse (see results in our previous work 29 ), this complementary method for bandgap determination based on photocurrent measurements was applied. From the photocurrent I ph versus time characteristics ( Fig. 4a and b) the spectral dependence of photocurrent was determined and presented in Fig. 4c. The plots of (I ph $ħn) 1/2 versus ħn (Fig. 4d) allow to determine bandgap value E from the intersection of the line tted to the experimental data with the photon energy hn axis. For TiO 2 and TiO 2 :Er,Yb thin lms, bandgap value from photoelectrochemical measurements is 2.93 eV and 2.97 eV respectively. For both methods, TiO 2 has slightly lower (0.04-0.05 eV) bandgap energy value than TiO 2 :-Er,Yb (1 at%, 1 at%). The difference of about 0.3 eV between the bandgap energy evaluated from optical and photoelectrochemical measurements was also observed in other work. 40 The photoluminescence spectra of crystallized and amorphous rare earth modied thin lms are presented in Fig. 5a (the pure TiO 2 thin lm served as a reference). In this experiment, Raman and PL signals are collected at the same time, thus both TiO 2 phonon bands (close to the 488 nm laser excitation line) and Er 3+ emission bands (at higher wavelengths) can be observed. A similar setup for the PL measurement of Eu 3+ /TiO 2 xerogels was described by Borlaf et al. 41 In the phonon bands range for crystalized TiO 2 :Er,Yb (1 at%, 1 at%) thin lm narrow lines at 490, 498, 501, and 504 nm can be observed, similarly to well-crystallized TiO 2 , while for amorphous TiO 2 :Er (10 at%) only increase in the noise level is observed. Detailed analysis of the 490-515 nm wavelength range is presented in Fig. 5b. The TiO 2 phases were identied according to the following reference Raman modes: 144 (E g ), 197 (E g ), 400 (B 1g ), 516 (A 1g + B 1g ) and 640 cm −1 (E g ) for anatase; 42 143 (B 1g ), 447 (E g ), 612 (A 1g ), and 826 cm −1 (B 2g ) for rutile; 43 154 (A 1g ), 247 (A 1g ), 320 (B 1g ), and 329 (B 2g ) cm −1 for brookite. 44 The obtained Raman shi values compared with the reference data conrm that the crystallized samples are a mixture of anatase, rutile and brookite ( Table 2). The recorded PL spectra (Fig. 5a) allowed for investigation of the luminescence properties and analysis of the optical quality of the thin lm. The sample with high Er 3+ activator content showed high emission in green, peaks with the main maximum at 524 and 547 nm, which corresponds to 2 H 11/2 / 4 I 15/2 , and 4 S 3/2 / 4 I 15/2 transitions in Er 3+ respectively, and weak emission in red with the maximum at 659 nm ( 4 F 9/2 / 4 I 15/2 ). Pure TiO 2 does not  exhibit additional emission peaks in the visible light range. The results of PL proved the lack of clustering within Er 3+ ions and the possibility of considering thin lms in energy conversion systems.
To analyse the upconversion properties of the photoanodes (rare earth modied TiO 2 thin lms on the ITO substrate), emission spectra were collected under the 980 nm laser diode excitation with laser pump power adjusted within the 100-1000 mW range (Fig. 6a-c). Measurements in a function of the pumping power conrmed the non-linear character of luminescence (Fig. 6d). All samples exhibit the main emission peak in the green wavelength range (l z 524 nm). TiO 2 :Er,Yb (2 at%, 10 at%) showed one order of magnitude higher green emission intensity than TiO 2 :Er,Yb (1 at%, 1 at%) and TiO 2 :Er (10 at%). The emission spectrum of the crystallized sample, similarly to the XAS spectra, has more split peaks, as presented in Table 2. The peak located at 490 nm wavelength in the UC spectra has origin from the second harmonic of the laser generated in the samples. On the basis of TiO 2 :Er and TiO 2 :Er,Yb spectra ( Fig. 6a-c), it was observed that the addition of a high concentration of Yb ions signicantly increases intensity of the upconversion emission. It occurs due to the following probable mechanisms 45 (schematic diagram of UC processes shown in Fig. 6e): (1) Due to two subsequent transfers (ET 1 , ET 2 ) from ytterbium ion ( 2 F 5/2 ) the 4 F 7/2 Er 3+ level is directly excited. Nonradiative relaxations from 4 F 7/2 Er 3+ level populate the following 2 H 11/2 , 2 H 11/2 and 4 F 9/2 levels. Then, radiative emissions at 524 nm, 546 nm and 658 nm occur as a result of transitions from 2 H 11/2 , 2 H 11/2 and 4 F 9/2 levels to the ground state 4 I 15/2 respectively.
(4) In the presence of high concentration of Er, Yb (2 at%, 10 at%) the third Yb photon (ET3) may excite 4 S 3/2 Er ions to the 2 G 9/2 level. Then, aer non-radiative relaxation to the 4 G 11/2 and 2 H 9/2 levels, emissions at 384 and 410 nm take place aer the transition to the ground state.
In case of higher than NaYF 4 phonon host like TiO 2 , both multi-phonon and cross-relaxation mechanisms are promoted. It is consistent with the results of log-log plots (l em = 524 nm, Fig. 6d) where the ET3 is possible only for the sample doped with 10 at% of Yb where slope exceeded 2.0 value. Increasing the total content of Ln 3+ ions changes luminous centers surroundings which is indicated by the GIXRD and XAS results. Moreover, the distances between Er 3+ and Yb 3+ ions are reduced as the Ln 3+ content increases, which increases UC probability and CR leading to enhanced UV-red emission.
Jung 26 reported that the main UC emission color of TiO 2 :Er 3+ /Yb 3+ phosphors is rather controlled by the Yb 3+ than the concentration of Er 3+with increasing Yb 3+ ions concentration the red-to-green ratio increased signicantly. Fig. 6d shows a logarithmic plot of the green emission intensities of rare earth modied thin lms as a function of the laser pump power. The slope values close to 2 (2.06, 1.76, and 1.80 mW) indicate that this level is populated in twophoton process, which is in accordance with Er 3+ energy diagram. 9 For TiO 2 :Er,Yb (2 at%, 10 at%) thin lm, UV (387 nm) and violet (410 nm) upconversion emission was observed (Fig. 6c inset), conrming the high efficiency of this UC material. Moreover, achieving upconversion from IR to UV is very attractive to enhance performance of TiO 2 based photoanodes. Based on energy diagrams in ref. 46-48 possible mechanism of three-photon upconversion in the Yb 3+ and Er 3+ couple is presented in Fig. 6e and can be described as follows. Aer two-photon excitation from the 4 I 15/2 ground state to the 4 F 7/2 excited state, multi-phonon relaxation to 4 F 9/2 occurs. Absorption of third 980 nm photon allows excitation to 2 G 7/2 , then relaxation to the 2 H 9/2 or 4 G 11/2 energy level takes place. During the 2 H 9/2 , 4 G 11/2 / 4 I 15/2 transition 410 and 387 nm photons are emitted.
The luminescence decay proles (Fig. 7) were measured under 980 nm excitation. The decay curves for green emission 2 H 11/2 / 4 I 15/2 (l em z 525 nm) were tted according to eqn (5): where I is intensity, ttime, s 1 , s 2 are decay times, A 1 , A 2 are tting constants. The values of the average decay time s were calculated as the weighted average: 49 For TiO 2 :Er and TiO 2 :Er,Yb thin lms, the average decay times s of green emission were several ms (0.90-2.49 ms) as shown in Fig. 7. Lakhotiya et al. 49 reported TiO 2 :Er thin lms sputtered and annealed at 350°C, which upon 800 nm laser excitation have a decay time of infrared emission ( 4 I 11/2 / 4 I 15/2 transition) around 10 ms. Zhanci et al. 50 showed that the measured lifetime of the erbium ion 4 S 3/2 and 2 H 11/2 states in oxyuoride tellurite glass at 303 K is approximately 20 ms.

Conclusions
The thin lm with small addition of activator and sensitizer TiO 2 :Er,Yb (1 at%, 1 at%) was a well-crystallized mixture of anatase, rutile and brookite, similarly to pure TiO 2 , while the high addition of activator or sensitizer ions in TiO 2 :Er (10 at%) and TiO 2 :Er,Yb (2 at%, 10 at%) thin lms led to amorphization as it was evaluated from GIXRD and XAS measurements. EDS and XAS measurements conrmed that erbium (activator) and ytterbium (sensitizer) ions were introduced into the TiO 2 host. The TiO 2 :Er and TiO 2 :Er,Yb thin lms deposited in the magnetron sputtering process showed upconversion from NIR to VIS with main emission in green (l em z 525 nm). Both TiO 2 :Er,Yb (1 at%, 1 at%) and TiO 2 :Er (10 at%) showed weak emission in red (l em z 660 nm), while for TiO 2 :Er,Yb (2 at%, 10 at%) thin lm increased red emission and even upconversion from NIR to UV was observed. Thus, it can be concluded that the higher content of activator and sensitizer allowed for more efficient upconversion, and the thin lm ability to upconvert NIR photons was not affected negatively by the amorphization. Both UC spectra and XAS spectra (Ti L 2,3 and O K edges) are more split for crystallized samples than for amorphous ones. The emission spectrum depends not only on the host type but also on the activator and sensitizer ions surroundings. As shown in XAS spectra, samples with high erbium (10 at%) and ytterbium (10 at%) content have different surroundings than sample with low Er and Yb addition. It has an impact on the average decay times of green emission. The average decay time of a several ms (typical for the TiO 2 host) decreases with increasing total Ln 3+ content from 2.49, through 1.59 to 0.90 ms. Further optimalization of thin lm composition, structure, microstructure, and optical properties to achieve high upconversion from NIR to UV and to maintain high photoelectrochemical performance is crucial for the development of efficient upconverting TiO 2 photoanodes for photoelectrochemical water splitting.

Conflicts of interest
There are no conicts to declare.